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        <h2 id="需求"><a href="#需求" class="headerlink" title="需求"></a>需求</h2><blockquote>
<p>下拉刷新某个主页面，需要刷新这个主页面里的所有子页面，每个子页面都是一个独立的模块，也就是要等到所有的模块都请求完成时，主页面的刷新才能结束,并执行一系列相应的操作。   </p>
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<p>以上就是一个完成的需求，OK，开始写代码，第一版的代码思路是这样的</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">@property</span> (<span class="keyword">nonatomic</span>, <span class="keyword">assign</span>) <span class="built_in">NSInteger</span> dataCallbackCount;</span><br><span class="line"></span><br><span class="line">- (<span class="type">void</span>)refreshView &#123;</span><br><span class="line">	<span class="keyword">self</span>.dataCallbackCount = <span class="number">0</span>;</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData0lWithComplete:^&#123;</span><br><span class="line">		<span class="keyword">self</span>.dataCallbackCount++;</span><br><span class="line">		[<span class="keyword">self</span> shouldEndRefresh:dataCallbackCount];</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData1lWithComplete:^&#123;</span><br><span class="line">		<span class="keyword">self</span>.dataCallbackCount++;</span><br><span class="line">		[<span class="keyword">self</span> shouldEndRefresh:dataCallbackCount];</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData2lWithComplete:^&#123;</span><br><span class="line">		<span class="keyword">self</span>.dataCallbackCount++;</span><br><span class="line">		[<span class="keyword">self</span> shouldEndRefresh:dataCallbackCount];</span><br><span class="line">	&#125;];</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">- (<span class="type">void</span>)shouldEndRefresh:(<span class="built_in">NSInteger</span>)count &#123;</span><br><span class="line">    <span class="keyword">if</span> (count &gt;= <span class="number">3</span>) &#123;</span><br><span class="line">        [<span class="keyword">self</span>.refreshView endRefresh];</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>第一版的代码有个问题，dataCallbackCount 属性和 shouldEndRefresh 方法 都是刷新方法内部才需要使用的，现在都变成类内部全局的了，其他的方法也可以使用这个属性和方法，而且 dataCallbackCount 还一直占用内存，这是不应该的。<br>我们知道block可以捕获变量，方法也可以用block代替。那么第二版改动如下：  </p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line">- (<span class="type">void</span>)refreshView &#123;</span><br><span class="line">    __block <span class="built_in">NSInteger</span> dataCallbackCount = <span class="number">0</span>;</span><br><span class="line">	 </span><br><span class="line">    <span class="type">void</span> (^shouldEndRefresh)(<span class="type">void</span>) = ^(<span class="type">void</span>) &#123;</span><br><span class="line">        <span class="keyword">if</span> (++dataCallbackCount &gt;= <span class="number">3</span>) &#123;</span><br><span class="line">            [<span class="keyword">self</span>.refreshView endRefresh];</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;;</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData0lWithComplete:^&#123;</span><br><span class="line">		shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData1lWithComplete:^&#123;</span><br><span class="line">		shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData2lWithComplete:^&#123;</span><br><span class="line">		shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>第二版我们将所有的代码都放到一个方法内了，同时也将 self.dataCallbackCount++; 放到了block内部，避免多次调用。dataCallbackCount 这个变量也会在结束刷新后释放内存，对于外部来说只需要调用 refreshView 就可以了，这样显得内聚很多。<br>但是仔细看的话，又会发现现在的 dataCallbackCount 也只是 shouldEndRefresh 内部需要使用而已 对于 [self.viewModel fetchxxx] 方法来说也应该是不可见的，但显然目前的这种写法是可见的。于是第三版改动如下：   </p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line">- (<span class="type">void</span>)refreshView &#123;</span><br><span class="line">    <span class="type">void</span> (^_shouldEndRefresh)(<span class="type">void</span>) = ^(<span class="built_in">NSInteger</span> dataCallbackCountMax) &#123;</span><br><span class="line">        __block <span class="built_in">NSInteger</span> dataCallbackCount = <span class="number">0</span>;</span><br><span class="line">        <span class="keyword">return</span> ^&#123;</span><br><span class="line">            <span class="keyword">if</span> (++dataCallbackCount &gt;= dataCallbackCountMax) &#123;</span><br><span class="line">                [<span class="keyword">self</span>.refreshView endRefresh];</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;;</span><br><span class="line">    &#125;(<span class="number">3</span>);</span><br><span class="line">    </span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData0lWithComplete:^&#123;</span><br><span class="line">		_shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData1lWithComplete:^&#123;</span><br><span class="line">		_shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData2lWithComplete:^&#123;</span><br><span class="line">		_shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>优化结束，如果需要优化的话可以将 [self.refreshView endRefresh]; 方法抽出来。最终代码如下： </p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line">- (<span class="type">void</span>)refreshViewWithCallBack:(<span class="type">void</span>(^)(<span class="type">void</span>))callback &#123;</span><br><span class="line">    <span class="type">void</span> (^_shouldEndRefresh)(<span class="type">void</span>) = ^(<span class="built_in">NSInteger</span> dataCallbackCountMax) &#123;</span><br><span class="line">        __block <span class="built_in">NSInteger</span> dataCallbackCount = <span class="number">0</span>;</span><br><span class="line">        <span class="keyword">return</span> ^&#123;</span><br><span class="line">            <span class="keyword">if</span> (++dataCallbackCount &gt;= dataCallbackCountMax) &#123;</span><br><span class="line">                callback ? callback() : <span class="literal">nil</span>;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;;</span><br><span class="line">    &#125;(<span class="number">3</span>);</span><br><span class="line">    </span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData0lWithComplete:^&#123;</span><br><span class="line">		_shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData1lWithComplete:^&#123;</span><br><span class="line">		_shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">	</span><br><span class="line">	[<span class="keyword">self</span>.viewModel fetchData2lWithComplete:^&#123;</span><br><span class="line">		_shouldEndRefresh();</span><br><span class="line">	&#125;];</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

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